The present invention is directed to an improved polishing pad for the chemical-mechanical planarization (CMP) of semiconductor wafers and a method of making it. Semiconductor wafers may have multiple layers of wiring devices on a single wafer. These wiring devices consist of hundreds of electrical circuits fabricated and interconnected in order to produce the computer chips that will eventually be die cut from the wafer. These wiring devices are called integrated circuits (IC). A layer of insulating materials, often silicon dioxide (S1O2), separates each layer of integrated circuits so that designated IC's interconnect. In order to pack more devices into less space, the requirements for feature size within the IC's has shrunk dramatically. There may now be feature sizes smaller than 0.01 microns. As layers of integrated circuits and insulating layers are deposited, one on the other, it is of utmost importance to maintain the wafer surface on each layer in an extremely flat condition. Features that make contact where not intended or do not make contact where intended can cause short circuits, open circuits and other defects that make a valuable product unusable.
The most effective method of planarizing multi-layer integrated circuit devices is chemical-mechanical planarization (often times called polishing), or CMP. When a layer of metal interconnects or insulation is put down, it must be polished flat; that is, it is planarized before the next layer is deposited. Otherwise, small surface irregularities may cause defects, and an extremely valuable part can be defective and lost. As each layer is deposited and planarized, multiple layers are successfully built up as needed for a particular device.
Chemical-mechanical planarization is superior to previously used technologies because it has proven capable of both local and global planarization of the materials used to build multi-level integrated circuit devices. In this process, a slurry of fine abrasive particles in conjunction with chemicals that attack the surface being polished are used together with a mechanical polishing process to achieve the necessary degree of flatness prior to the deposition of the next layer.
One problem with this approach has been changes in the rate of removal over the life of the polishing pad. Most conventional polishing pads in use at present consist of polyurethane-cast resin, polyurethane fibers impregnated with polyurethane, or a combination thereof. The polishing surface of these pads tends to become glazed and worn over time during the polishing operation on multiple wafers. This changes the pad's surface characteristics sufficiently to cause the polishing performance to deteriorate significantly over time. This has been overcome by conditioning the pad surface during use, or between wafers as needed. This conditioning procedure removes the glazed worn surface from the pad and restores polishing pad performance.
The major reason conventional polyurethane and other thermoplastic-based polishing pads require pad conditioning is that the surface of these pads undergoes plastic deformation during use. This is commonly called creep, and it is a common occurrence when thermoplastic materials are subjected to heat and pressure, however slight. Additionally, abrasives from the polishing slurry and other polishing debris embed themselves in the soft surface of the thermoplastic polishing pad thus contributing to surface deteriorating and glazing. This has been overcome in the semiconductor industry by pad conditioning. Pad conditioning renews the pad surface during polishing operations as required to restore original pad performance before this performance falls below acceptable levels. Some operations require continuous pad conditioning, others intermittent, some between wafers. Most semiconductor wafer polishing equipment includes a pad conditioning apparatus built into the equipment. This pad conditioning apparatus generally consists of an arm to which is attached a rotating spindle to which is attached the conditioning disk. This conditioning disk generally consists of fine diamond grit bonded to the bottom surface of the disk. When needed, the conditioning disk traverses the polishing pad, renewing the polishing pad surface and restoring polishing pad performance. Unfortunately, pad conditioning actually removes material from the polishing pad surface so that over time the polishing pad is slowly worn away, thus shortening the polishing pad's life.
Another problem with pad conditioning systems is the cost of maintenance and the cost of the diamond conditioning disks. In addition, diamond particles sometimes break loose from the conditioning disk and cause scratches on the wafer that cannot be repaired, adding to the cost of ownership. Since pad conditioning reduces pad life and increases time lost for more frequent pad replacement, it is obvious that reducing the need for and/or the amount of material removed during pad conditioning with the attendant reduction in cost of ownership is a very desirable goal.
Prior-art polishing pads are often formed with asperities on the polishing surface of the pad. In these prior art polishing pads this type of asperity is plastically deformed by polishing action and/or constantly worn away by the conditioning action. In order to renew the surface (maintain the original surface structure) the pad is conditioned during use. This can be considered an in-situ grinding operation. Conditioning disks can be compared to the round sanding disks commonly used on portable hand drills. The grit, however, on a conditioning disk consists of fine diamond particles as the active conditioning (grinding or sanding) surface. Thin surface layers of the polishing pad is continuously removed from the pads surface in order to renew the asperities. Due to this removal, the life of the polishing pad is shortened accordingly.
As noted above, all prior-art polishing pads for use in CMP processes require either periodic or continuous pad-conditioning for refreshing and renewing the polishing process. Pad-conditioning is typically accomplished by use of a conditioning disk consisting of a surface having abrasive grit of diamond or cubic boron nitride that removes the outer, spent polishing layer of the polishing pad. However, pad-conditioning removes an amount of material from the polishing layer that may considerably shorten the life of the pad for in the CMP-process polishing of substrates. These conditioning disks need to be periodically replaced, when it has been determined that the conditioning of the polishing pads falls below a desired or required value. The life of a conditioning disk is dependent upon the type of wafer the polishing pad is polishing, the force exerted against the conditioning disk during pad-conditioning, as well as other factors. For example, for the CMP polishing of tungsten, which requires the use of a polishing slurry containing very abrasive particles, a conditioning disk will—with all other things being equal—have a shorter life-span owing to the greater degree of abrasiveness of the abrasive particles of the polishing slurry which would cause greater wear of the diamond grit of the conditioning disk. Determination as to when to replace a conditioning disk may be based on the simple the determination that the polishing pads no longer polish wafers to the required specifications. Alternatively, an objective measure may be employed, such as that disclosed in U.S. Pat. No. 6,368,198—Easter, et al., where the current drain on the conditioning-disk motor or on the polishing-pad platen motor during the pad-conditioning process has been measured to have increased to a preset, prescribed limit indicative of unacceptable conditioning-disk wear.
In above-mentioned parent application Ser. No. 10/087,223, the polishing pad thereof is preferably used in those environments where polishing-pad conditioning is not generally or typically required. The present invention is directed to the use of the polishing pad of above-mentioned parent application Ser. No. 10/087,223 in those environments where polishing-pad conditioning is either necessary or, at times, desirable. The polishing pad of the present invention exhibits improved resistance to the conditioning process used to maintain performance in the chemical mechanical planarization of semiconductor wafers and similar materials, particularly silicon dioxide. Reduced conditioning, more specifically, reduced amount of pad material removed in the conditioning process, results in longer lived polishing pads, reduced down time due to less frequent pad replacement and longer lived diamond conditioning disks. The net result is a significant reduction in the cost of consumables. The use of abrasive particles, such as alumina and silica, are known to have been used in CMP slurries for achieving the polishing of the substrate. These abrasive particles may also be imbedded in the polishing pad itself, and are used to enhance and improve the consistency of the polishing of the substrate during the CMP polishing process. These abrasive particles are typically used in a self-dressing type of polishing pad, which continually exposes particles to the substrate being polished. These abrasive particles are of a size generally described as being millimeter-sized. Examples of such prior-art polishing pads with millimeter-sized abrasive particles are disclosed in U.S. Pat. No. 6,022,264—Cook, et al., and U.S. Pat. No. 6,299,516—Tolles.
It is also known to use nanometer-sized particles, such as silicon dioxide, alumina, and the like, to precondition a polishing pad before first use in the polishing of the substrate during the CMP polishing process. The nanometer-sized particles are contained in a gas that is injected against the polishing surface of the polishing pad by a nozzle. An example of such a slurry is shown in U.S. Pat. No. 6,300,247—Prabhu.